The present invention relates to a reciprocating color printer such as a plane-scanning type color printer.
Recently, plane-scanning color printers have been described for use as printers having a printing head for printing an image or character on a recording sheet by reciprocation, as exemplified by description in the Japanese Patent Provisional Publication No. 64-75255. Conventional plane-scanning type color printing systems will briefly be described hereinbelow with reference to FIGS. 1 to 3. In FIG. 1, the plane-scanning type color printer is equipped with four printing heads (yellow, magenta, cyan and black printing heads) illustrated at characters A to D, each of which has N ink-discharging nozzles and is arranged to be mounted on an appropriate carriage, not shown, so as to allow reciprocation. A recording sheet illustrated at character P is fed in a direction indicated by an arrow Y and the printing heads A to D are movable in directions (going and returning directions) indicated by an arrow X which is perpendicular to the direction Y. Each printing head discharges ink in accordance with the movement in the going direction, i.e., from the left side to the right side in the illustration, so as to color-print N lines as illustrated in FIG. 2. In response to completion of the N-line color printing, the recording sheet is fed by an amount corresponding to the N lines for the returning-direction color printing. Thereafter, the printing head similarly discharges ink in accordance with the movement in the returning direction, i.e., from the right side to the left side in the illustration, so as to color-print N .lines as illustrated in FIG. 2.
In FIG. 3, another conventional plane-scanning type color printer is similarly equipped with printing heads A to D each of which has N ink-discharging nozzles which are successively arranged at a predetermined interval which is twice the line-printing pitch (half density), where the term "line-printing pitch" refers to the pitch or distance between, of lines to be finally printed by the printing heads for printing images or characters on a surface of a writing sheet. When moving in the going direction, i.e., from the left side to the right side in the illustration, each printing head performs the color-printing at the location of every other line to be printed, i.e., prints at half density, so as to print N (first to Nth) lines. That is, the first to Nth lines are printed by the first to Nth ink-discharging nozzles of the printing head so as to leave a separation for printing one line therebetween. In response to completion of the going-direction printing, the recording sheet P is moved by a distance corresponding to one print line and the printing head moves in the returning direction so as to newly color-print N additional lines between the first to Nth lines formed by the going-direction printing due to the same printing head.
There is a problem which arises with the former, however, in that a considerable tone difference occurs between the going-direction printing and the returning-direction printing because the order of overlapping inks in the going-direction printing is reversed with respect to the order of overlapping inks in the returning-direction printing. On the other hand, in the case of the latter, the tone difference problem is substantially resolved because the going-direction printing and the returning-direction printing are alternately performed on every other line, whereas, because the print lines due to the same ink-discharging nozzle are in close relation to each other, the difference of the ink-discharging amount between the nozzles greatly affects the printing quality.